In the evaluation of earth boreholes drilled in earth formations to produce hydrocarbons, determination of the porosity of the formations is considered essential for decision making. Nuclear magnetic resonance ("NMR") provides a means of measuring total and producible porosity of earth formations. In certain conditions NMR well logging can provide important information on the pore size of formation rock and on the type of fluid contained therein. Measurement of nuclear resonance requires a static magnetic field B.sub.0 and a radio frequency (RF) magnetic field in the earth formation that is being probed. [As used herein, an RF field generally has a frequency in the range 2 KHz to 10 MHz.] Atomic nuclei with a nonzero nuclear magnetic moment and spin angular momentum precess about the static field B.sub.0 with an angular frequency .omega..sub.0 =.gamma.B.sub.0 when perturbed from their thermal equilibrium. The constant .gamma. is the gyromagnetic ratio of the resonating nucleus, most commonly the hydrogen nucleus. For hydrogen nuclei, the gyromagnetic ratio is 2.675198775.times.10.sup.8 radian/second/Tesla. To manipulate the spin state of the particles, for example, to perturb the thermal equilibrium, a radio frequency (RF) magnetic field B.sub.1 is needed. The frequency of the RF field B.sub.1 should be close to .omega..sub.0 and substantially perpendicular to the static field B.sub.0 in the region of investigation. Magnetic resonance is observed by detecting the oscillating magnetic field produced by the precession of the spins. Typically, but not necessarily, the same coil that produces the RF field B.sub.1 is used for detection. In pulsed-NMR, repeated pulses are applied to the coil and spin-echoes are detected in between the transmitted pulses. Reference can be made, for example, to U.S. Pat. Nos. 5,376,884, 5,055,788, 5,055,787, 5,023,551, 4,933,638, and 4,350,955 with regard to known nuclear magnetic resonance logging techniques.
In logging-while-drilling, the measurement apparatus is mounted on a drill collar. Drill collars are long, tubular pieces of a strong material, typically nonmagnetic stainless-steel. Drill collars and drill pipes transmit the torque from the surface apparatus to the drill bit. During drilling, the drill collars typically rotate about their axes, which are substantially aligned with the axis of the borehole. The rates of rotation of the drill collars and the drill bit are the same in rotary drilling, and can be different if a downhole mud motor is used. In either case, the drill collar is subject to rotation. For logging-while-drilling NMR logging, the magnitudes of B.sub.0, B.sub.1, and the angle between them should be substantially invariant of the rotation angle in the region of investigation. This does not preclude the possibility that the directions of B.sub.0 and B.sub.1 may depend on the rotation angle. The foregoing invariance is required because magnetic resonance measurements take on the order of 0.01 to 1 seconds during which the drill collar may rotate by a substantial angle. Consistent preparation and measurement of spin states are not possible without the rotational invariance.
Directional drilling involves the drilling of a well bore along a deviated course in order to reach a target region at a particular vertical and horizontal distance from the original surface location. Directional drilling is employed, for example, to obtain an appropriate well bore trajectory into an oil producing formation bed (or "pay zone") and then drill substantially within the pay zone. A horizontally drilled well can greatly increase the borehole volume in the pay zone with attendant increase in oil production. Recent advances in directional drilling equipment and techniques have greatly improved the accuracy with which drilling paths can be directed.
Nuclear magnetic resonance logging systems have previously been proposed for logging-while-drilling applications. If an NMR logging device of a logging-while-drilling system has an axially symmetric response, the NMR characteristics measured by the logging device will tend to average the signals received circumferentially from the formations. For example, when drilling a near-horizontal well along the boundary between two formation beds with dissimilar producible porosities, such a logging device would give indication of an intermediate porosity. It would be very advantageous to be able to use NMR to better delineate the presence, locations, and characteristics of the formation beds in this type of a situation.
It is among the objects of the present invention to address limitations of the prior art with regard to nuclear magnetic resonance logging techniques and apparatus.